1. Field of the Invention
This invention relates to an angle detector such as a rotary encoder for detecting rotation angle, particularly to an angle detector provided with self-calibration capability that enables calculation of calibration values for scale lines that include angle data error owing to attachment eccentricity in the use environment, angle detector aging, and the like.
2. Description of the Prior Art
Rotary encoders and similar angle detectors operate on the general principle of using a read head to count scale lines formed at the periphery of a circular disk and output the counted value as angle data. Various devices are in use such as those shown in FIGS. 6 and 7. Since the scale lines of an angle detector are created artificially, they are not exactly equiangular, so that errors occur in the angle data obtained from the positions of the scale lines. The radial straight lines in FIG. 8 represent ideal scale line positions (equiangularly spaced lines) and the radial short broken lines indicate actual scale line positions. A plot of the deviation from the ideal positions is shown in the graph on the right side of FIG. 8.
The points in the graph of FIG. 8 are the calibration values of the angle detector scale lines. The number of scale lines in the drawing is only 36. An actual angle detector has from several thousand to several hundred of thousand scale lines. Among the various methods of calibrating the lines are included a number of methods that achieve self-calibration by mutually comparing the scale lines of two angle detectors. These methods enable simultaneous calibration of two previously uncalibrated angle detectors and therefore do not require use of an angle detector of higher-order accuracy. The point of “self-calibration” is that even by comparing two unknown angle detectors it is possible to calibration values for both simultaneously.
FIG. 9 shows the Japanese national standard instrument for angle measurement which is kept in the inventors' Institute. It is an instrument for calibrating an angle detector located inside the angle measurement instrument and an angle detector to be calibrated positioned thereabove by the self-calibration method. The self-calibration method used is the equal division averaged method.
A simplified version of the equal division averaged method will be briefly explained with reference to FIG. 10. The difference between the angle graduation signal of one of multiple first scale read heads 12, 12 . . . installed in a first angle detector 11 and the angle graduation signal of a second read head 14 installed in a second angle detector 13 located above the first angle detector 11 is measured (difference Sa1). Next, the difference between the angle graduation signal of an adjacent first scale read head 12 in the first angle detector 11 and the second read head 14 in second angle detector 13 (difference Sa2) is similarly measured. The angle graduation signal differences between the remaining first scale read heads 12 are similarly read and the average value SaAV of the measured differences (Sa1, Sa2, Sa3, Sa4, Sa5) is calculated and, optionally, the value of Sa1−SaAV is calculated. A calibration curve for the second angle detector 13 can then be obtained from either of these values. When five first scale read heads 12 are installed, they are disposed at angular spacing equal to one-fifth of a full circle (360 degrees). When N number of first scale read heads 12 are installed, they are equiangularly spaced at 1/N of a full circle.
JP-A 2003-262518 teaches incorporation of self-calibration capability in a single angle detector. The angle detector uses the multi-reproduction head method of self-calibration. JP-A HEI 10-300053 teaches size reduction of an angle detector of the type shown in FIG. 10.
As shown in FIG. 10, the rotational shafts of the first angle detector and the second angle detector to be calibrated are coaxially coupled by a coupling 15. However, the coaxiality is generally not perfect. Namely, some amount of eccentricity is present that cannot be neglected. The effect of the axial eccentricity is numerically calculated from the calibration value and the effect obtained therefrom is subtracted to obtain a calibration such as shown in FIG. 8. However, when the calibrated angle detector is detached and installed in the instrument in which it is actually used, its rotational shaft is coaxially coupled with the rotational shaft of the instrument by means of a similar coupling. At this time, the amount of eccentricity and the effect thereof cannot be estimated.
The calibration curves of FIG. 11 were plotted using actual data obtained for the angle detector of FIG. 7. Curve A represents the amount of deviation of the scale lines of the angle detector from the ideal scale line positions. The curve looks continuous owing to the large number of scale lines (18,000). Curve B is a calibration curve measured for the eccentricity with respect to the mating axis (axis of the angle detector in the calibrator). Curve C represents the effect of the eccentricity. The curves are interrelated such that Curve C+Curve A=Curve B. When the angle detector is installed in an instrument for use, the effect of eccentricity corresponding to Curve C cannot be detected. Moreover, the instrument shown in FIG. 10 has a problem in that its structure limits the amount of size reduction possible.
The object of the present invention is therefore mainly to provide an angle detector with self-calibration capability which avoids occurrence of error owing to coupling with the rotational shaft of the angle detector in a calibrator by enabling calculation of calibration values for scale lines that include angle data error owing to the effect of eccentricity of the angle detector itself and factors such as angle detector aging, thereby ensuring accurate calibration at all times, and which can be reduced in size.